Hi:
I want to know if I can match transistor using the Hfe meter of my cheap
multimeter.
If not, can anybody tell me a easy way to do it.
I need to match about 6 transistor for a transistor ladder filter, and I think
it will be useful knowledge for future projects.
Thanks.

Did try some googling on this but can't really find what characteristic trannies should be matched on for ladder filters. I'll try to write down what I think to know, that will be a bit general though

A multimeter doing Hfe measurement will do so by applying a fixed base current and measuring the resulting collector current. Usually though matching should be checked over a wide range of signal levels. Otoh for filters this is usually considered less important than for oscillators (unless you want to build tracking oscillating filters, but then a ladder filter seems less ideal than let's say a state variable filter).

What can be said is that Hfe matching probably will be a good first order approximation for matching, in that when the Hfe-s differ wildly other parameter will not match good either.

So I guess you can get away with the Hfe measurement on your meter. But I'm not really very sure, as I don't know how important the Vbe vs Ib characteristic is for a ladder filter. to measure that relation Hfe measurement is not directly useful as I think this relation varies with the size of the base emiter junction - I read that somewhere - and that would depend on manufacturing precision ...

There is a (well-known) mistake in the second schematic. Add a 10k resistor between the transistors as in the first schematic.

One other point -- if you buy a modern batch of devices they will usually all be matched to within ~2 mV, in my experience. If you get a late array such as the CA3083 matching is usually better than .2 mV.

For accurate matching for critical applications such as roll-your-own OTA's, you can use bridge-type circuits. With care, you can measure within .01 mV.

There is a (well-known) mistake in the second schematic. Add a 10k resistor between the transistors as in the first schematic.

luckily i had to use the upper one.
Thanks for the tip.

dave_b wrote:

Perhaps a silly question: Regarding the Moog test circuits, is it necessary to remove power when swapping transistors?

No, good question.

I did this things without any knowledge. sometimes thats better, sometimes not .
One should not touch the Transistors by Hand when matching because of heat , so i used a plier.
This tourned out as not so easy and very Timeconsuming. ( really Timeconsuming )

Once i took the Transistor on its legs, crosswise, so i did a short. To be save, i throwed it away.
Next: to allways pull down the Power would be also very Timeconsuming.( The power i use is a long Powercord for Modules, coming from my modularrack )
I did not

Am I right in reading that one can use the Moog matcher at +-15V without changing any resistor?

The 100 uA current is determined by the voltage divider at the opamps input and the 51 k resistor going to the neg. supply. The opamps + input is set at -5V for a 10V supply, it will try to regulate it's output in such a way that the - input will also be at -5V. This means there is 5V over the 51 k resistor and that means that about 100 (98) uA will flow.

For a 15 V supply the voltage at the opamp's + input wil be -7.5V resulting in 7.5 V to be present over the 51 k resistor resulting in approx. 150 uA of current to flow. So you'll measure at another set point then.

When you want that 100 uA again the resistor should be around 75 k for a +/- 15 V supply. the current does not really depend on the positive supply voltage, but it's linear with the negative supply.

You could also replace the 10 k resistor going from the opamp + input to the negative supply with a 5V1 zener, that would make the circuit independent from the negative supply voltage as well. A 5v1 zener coincidentally also has a near zero temperature coefficient. In that case keep the 51 k resistor._________________Jan

Am I right in reading that one can use the Moog matcher at +-15V without changing any resistor?

The 100 uA current is determined by the voltage divider at the opamps input and the 51 k resistor going to the neg. supply. The opamps + input is set at -5V for a 10V supply, it will try to regulate it's output in such a way that the - input will also be at -5V. This means there is 5V over the 51 k resistor and that means that about 100 (98) uA will flow.

For a 15 V supply the voltage at the opamp's + input wil be -7.5V resulting in 7.5 V to be present over the 51 k resistor resulting in approx. 150 uA of current to flow. So you'll measure at another set point then.

When you want that 100 uA again the resistor should be around 75 k for a +/- 15 V supply. the current does not really depend on the positive supply voltage, but it's linear with the negative supply.

You could also replace the 10 k resistor going from the opamp + input to the negative supply with a 5V1 zener, that would make the circuit independent from the negative supply voltage as well. A 5v1 zener coincidentally also has a near zero temperature coefficient. In that case keep the 51 k resistor.

I think it´s right there by looking at the symmetry of both circuits. In the NPN circuit the reference transistor is between 51k and 10k resistors. In the PNP the 10k is missing.

Yes, right. Sorry not to be clearer. If you look (literally) between the two transistors on the schematic you will see that one circuit has a 10 k resistor and the other one has just a wire. Put the additional 10k where the wire is.

If you've done any DIY lately you've likely discovered that it is becoming more difficult to find thru-hole matched transistor pairs. The solution is either to use SMT (and the SMT transistor pairs are almost microscopic!) or to resort to hand-matching discreet transistors.

This thread has some test circuits to allow one to match transistors. It will be useful to me and I decided to bump this thread for others to see._________________-- Kevinhttp://kevinkissinger.com

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